Selection started from nested table causes an error in a browser while scrolling down

[mlewand]

Are you reporting a feature or a bug?

Bug

Check if the issue is already reported

http://dev.ckeditor.com/ticket/17052

Provide detailed reproduction steps (if any)

1. Create a 5x5 table (editor should become scrollable).
2. In the first cell make 2x2 nested table with width set to 15.
3. Fill every cell with letter "s".
4. Start selection from nested table, then move it out of the table and drag it to scroll down the editor.

Expected result

Selection should stay in a nested table without returning any errors in console.

Actual result

There is an error in a console. https://gist.github.com/Mgsy/b5c95910313a7cb70e87beae921ac74f

Other details

Windows 10, tested on Chrome, Edge, IE11, Firefox.

[mlewand]

Fix proposed in #406.